1. Field of the Invention
The present invention relates to an image developing method used in an electrophotographic image forming device and for developing, by using a developing agent, a latent image on a latent image carrier, an image developing device using the image developing method, and an image forming device having the image developing device.
2. Description of the Related Art
In an electrophotographic image forming device of the related art, image data are optically recorded on a latent image carrier (for example, a photo-conductive object, below, referred to as a “photoconductor”), which is uniformly charged in advance, and an electrostatic latent image is obtained by exposure of reflected light from a manuscript, or by writing with a light writing device; the electrostatic latent image is converted into a visible image by toner (a toner image), which serves as a developing agent and is provided from a developing agent carrier (such as a developing roller) of a developing device. The visible toner image is transferred to a transferring member, such as a transfer sheet, and is fused on the transfer sheet, thereby forming an image as desired. In addition, in order to remove toner remaining on the photoconductor after image transfer, a cleaning unit is provided on a downstream side of the image transfer position.
Developing methods used by the developing device in the image forming device include single-component developing in which only the toner is used as the developing agent, and two-component developing in which the toner and magnetic carriers are used as the developing agent. In an image forming device of the related art having a developing device using single-component developing, the toner on the developing agent carrier (a developing roller) may form a film over time. Below, formation of a toner film is referred to as “toner filming”. When toner filming occurs, charging characteristics of the toner become unstable, and this causes stains on the background, reduction in density of an image, or other abnormal images. Specifically, toner filming on the surface of the developing roller indicates that because a film-like toner layer covers the surface of the developing roller, it becomes difficult to charge the toner with the surface of the developing roller, and this makes the charging characteristics of the toner becomes unstable.
The mechanism of occurrence of the toner filming is as below.
When only a part of a toner-carrying width of the developing roller, is used for continuous image formation, for example, when the right half of an image is white, the toner in the region outside of the image forming portion (below, the region out of the image forming portion is referred to as “non-image portion”) has nothing to do with developing in a series of developing operations, and merely adheres to the surface of the developing roller by the Coulomb force. In addition, since an end of a regulating blade, which is used for regulating the thickness of the toner on the surface of the developing roller to make the toner layer thin, is arranged to slidably contact the surface of the developing roller, the toner adhering to the non-image portion of the developing roller is repeatedly pressed by the regulating blade against the surface of the developing roller once the regulating blade passes through. Due to the pressure of the regulating blade, part of the toner consistently adheres to the surface of the developing roller and is pressed by the regulating blade many times. Note that some of the toner is scraped off the developing roller by the pressure of the regulating blade, and some toner newly adheres to the surface of the developing roller to be repeatedly pressed by the regulating blade.
In this way, when the toner, which continuously adheres to the surface of the developing roller and is pressed by the regulating blade many times, receives the pressure from the regulating blade, due to the heat generated by the pressure, the toner becomes film-like and can easily adhere to the surface of the developing roller. Such toner adhering to the surface of the developing roller further repeatedly receives the pressure from the regulating blade; thus, the amount of charge of the adhering toner increases, and thereby, it becomes more and more difficult for the adhering toner to be separated from the surface of the developing roller.
In the phenomenon called toner filming, the toner adhering to the non-image portion of the developing roller is repeatedly pressed by the regulating blade, cannot be separated from the surface of the developing roller, and thus forms a toner film.
The same problem occurs in an image forming device having a developing device using two-component developing.
Further, due to stirring by a toner conveyance member and a stirring member in the developing device, an additive agent, which is added in the developing agent to improve fluidity, may be berried in the toner, or be striped, and this degrades the charging characteristics of the toner.
For example, Japanese Laid Open Patent Application No. 9-258553 (hereinafter, referred to as “reference 1”) discloses a technique for preventing toner filming on the developing roller. This technique includes performing a flash operation to remove the toner on the developing agent carrier by moving the toner on the developing agent carrier to the latent image carrier; because of the flash operation, the toner on the developing agent carrier is moved to the latent image carrier, and is used or is returned to the developing device.
In addition, Japanese Patent Gazette No. 3029648 (hereinafter, referred to as “reference 2”) discloses a technique in which a printing rate is calculated for each manuscript; when the printing rate is less than a certain value, a checked pattern of the developing agent having a size corresponding to the ratio of the printing rate is formed before the next image formation operation, thus compulsively consuming the developing agent.
In the technique disclosed in reference 1, in order to prevent reduction of toner yield, as described above, the flash operation is performed to recycle the removed toner. However, by the technique disclosed in reference 1, it is difficult to prevent degradation of toner caused by mechanical stress in such as stirring, developing, and recollection operations, and toner recycling may cause degradation of image quality. Further, considering color copying, in the technique disclosed in reference 1, it is supposed that the flash operation is performed between two pieces of paper printed consecutively; however, because it is necessary to perform reverse transfer from the transfer unit for color mixing in a color copier, it is difficult to apply the technique disclosed in reference 1 to a color copier.
In addition, for purposes of maintenance, such as toner filling up, it is required to show users the amount of residual toner or the consumed toner. Generally, an optical sensor is provided in a toner container to detect the amount of the residual toner. However, in a color copier, it is necessary to provide a sensor for each color, and thus the number of parts rises, the cost of the device increases; and it is difficult to reduce the size of the device.